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Two ERP components are discussed in relation to studies of semantic and mnemonic processing, namely the N400 and LPC. The N400 is a monophasic negativity typically observed between 250 - 500 ms post-stimulus. Semantic and repetition priming studies utilising linguistic stimuli in the visual modality (i.e. words, nonwords), and stimulus paradigms (i.e. sentence priming and lexical decision tasks) have indicated that the N400 is readily evoked by semantic anomaly and is sensitive to word frequency (low frequency words eliciting greater amplitude N400s than high frequency words), word class (larger N400s to content words), semantic relatedness (larger N400s to unrelated words), subject expectancy (larger N400s to unexpected words within a context), phonology (larger N400s to non-rhyming words and nonwords), and repetition (attenuated N400s following word repetition). Theoretical formulations suggest the N400 indexes the degree of spreading activation throughout the semantic network (Morton, 1969), contextual integration (Rugg, 1990), semantic expectancy (Kutas & Hillyard, 1984), or a memory search process (Bentin & McCarthy, 1994). Which of these formulations most accurately explains the N400 is currently unresolved. Whether the negativity observed following semantic anomaly in paradigms employing nonlinguistic stimuli (e.g. pictures, faces, and music) is reflecting the same process as the N400 elicited by linguistic anomaly, is also the subject of considerable debate. Enhanced LPC amplitudes in language tasks are typically recorded to sentence final words. The broad, post N400 positivity, occurs approximately 550-800 ms post-stimulus and is presumed to reflect processes associated with closure (Friedman, Simson, Ritter, & Rapin, 1975; Kutas & Hillyard, 1982), certainty (Stuss, Picton, Cerri, Leech, & Stethem, 1992), and integrative elaborative processing (Andrews, Mitchell, & Ward, 1993). All task relevant stimuli appear to elicit the LPC, its amplitude being inversely related to subjective probability. The LPC is also associated with certain aspects of mnemonic processing. Enhanced LPC amplitudes have been recorded to stimuli which are subsequently recognised, to 'seen' stimuli as compared to 'unseen', and to the second presentations of stimuli. Subsequent to these findings, it has been hypothesised that the LPC observed in memory and repetition paradigms reflects some process associated with both encoding and retrieval. Resulting from the perceived similarity between the LPC component elicited in these various paradigms, some investigators posit that similar episodic processes subserve them (Besson & Kutas, 1993). |
